Chemical vapor deposition (CVD) is a well known technique for obtaining very pure metal coatings. The chemical vapor deposition process (particularly for depositing nickel) has unparalleled ability to deposit uniform coatings. CVD coatings do not build up on external corners, edges or projections, as do electroplated deposits. And it has outstanding ability to deposit into internal corners and recesses, which electroplated coatings are seriously deficient in accomplishing.
Electroless coatings are uniform; however, electroless nickel is brittle and is not pure (contains 2-8% phosphorus or boron), limiting its usefulness. Copper is the only other metal widely deposited electrolessly, primarily on printed circuit boards.
Vacuum metallized coatings are deposited on a line-of-sight; and flame sprayed coatings are applied on a line-of-sight. Thus uniformity can never compare to CVD. Basically, in CVD, the vapor of a heat decomposable compound of the desired coating metal is allowed to contact the article to be coated (the substrate) under air-excluding (i.e., nonoxidizing) conditions at a temperature sufficiently high to cause the coating metal compound or precursor to decompose. Various types of heat-decomposable compounds are known in the art. Metal carbonyls are particularly desirable as coating agents in CVD processes because of their low decomposition temperatures, which makes possible the use of low operating temperatures that do not damage the substrate and which achieve substantial reduction of technical and engineering problems with attendant economies of operation.
Low operating temperatures particularly make it possible to use equipment having silicone or fluoropolymer rubber seals, both of which are effective and readily available but unable to withstand operating temperatures above 275.degree. C. At operating temperatures of 300.degree. C. or higher, the choice of sealing materials is very limited. No commercial elastomers are available that stand the heat.
Metal carbonyls have two limitations as CVD coating agents: (1) not all metals which are desirable coating metals form carbonyls; and (2) certain metals which are usable as substrates cannot be adherently coated directly by CVD at temperatures below about 300.degree. C. using a metal carbonyl as the vapor coating agent. Notable among such substrate metals are ferrous metals (iron, nickel, cobalt) and their alloys. The reason that these substrate metals cannot be adherently coated by CVD using a metal carbonyl is that the substrate metals chemisorb carbon monoxide, i.e., they tenaciously chemisorb appreciable commodities of carbon monoxide under air-excluding conditions and at all temperatures. Carbon monoxide is inevitably released as a metal carbonyl decomposes on the substrate surface and is adsorbed before the substrate acquires a metal coating. How this inteferes is explained in the following paragraphs. For brevity, metals which absorb appreciable quantities of carbon monoxide under such conditions will be referred to as "metals which significantly chemisorb carbon monoxide", and conversely, metals which adsorb carbon monoxide weakly or not at all under such conditions will be referred to as "metals which do not significantly chemisorb carbon monoxide" or as "metals which have little or no tendency to chemisorb carbon monoxide".
Aluminum and its alloys are not directly coatable adherently by CVD from a carbonyl because they have an oxide layer not reducible at reasonable temperatures. Brass and zinc are not coatable adherently from a carbonyl because the zinc metal has a low vapor pressure, and off-gases. However, these metals when coated adherently with the selected undercoat metals according to this invention can then be coated adherently by CVD from a metal carbonyl.
The term, "chemisorption" and related words including "chemisorb" are used in their art-recognized meaning, as defined for example in "The Condensed Chemical Dictionary", 10th edition, 1981, page 225; revised by G. G. Hawley, published by Van Nostrand Reinhold Co., New York. As explained in "The Condensed Chemical Dictionary", chemisorption results in the formation of bonds comparable in strength to ordinary chemical bonds, between the adsorbent (usually a solid and typically a metal) and the gas or liquid coming into contact with it. The bonds formed in chemisorption are much stronger than the VanderWaals bonds characterized in physical adsorption, or "physisorption", as it will be called herein. While physisorption is reversible, chemisorption is not; the chemisorption material is released only by appreciable expenditure of energy, and then often is released as a material different from that which was chemisorbed.
The mechanism by which an adherent bond is formed between the coating metal and a substrate metal which chemisorbs carbon monoxide is believed to be as follows: at the surface where decomposition of the metal carbonyl takes place, initially, the metal carbonyl molecule splits apart and the carbon monoxide so formed becomes adsorbed on the substrate metal surface before the coating metal atom can be attached to the crystal lattice of the substrate. Then, when the metal atoms approach the crystal lattice, the adsorbed carbon monoxide is released and a gas-filled gap is created between the substrate and the deposited atoms. Once CVD has started and is proceeding smoothly, the mechanism of deposition alters, the metal carbonyl molecule is first adsorbed on the substrate coating and then decomposes with the carbon monoxide escaping and the metal atom fitting into the crystal lattice of the now growing coating. Even so, there is competition between adsorption of carbon monoxide and the metal carbonyl. Furthermore, the gas filled gap at the interface between the substrate and the coating remains. As a consequence, the bond between the substrate and the coating is only a mechanical bond and not a metallurgical bond. A metallurgical bond can be distinguished from a mechanical bond by a simple bending test when both the substrate and the coating metal are ductile. A coated strip of substrate metal is bent around a radius equal to its thickness; the coating will not separate if there is a metallurgical bond between the substrate and the coating metal. If the bond is simply a mechanical bond, the coating will separate when this bending test is carried out. It is believed that there is some attractive force between atoms of the substrate metal and atoms of the coating metal at the interface in a metallurgical bond; a mechanical bond, on the other hand is achieved simply by interlocking of the coating metal with surface asperities of the substrate.
However, when deposition is on a metal that weakly or not at all chemisorbs carbon monoxide (copper, tin, silver, gold), very little or no carbon monoxide is chemisorbed, and some may be physisorbed; the physisorbed (and weakly chemisorbed) carbon monoxide is released simultaneously as the atoms of metal are deposited, and escapes while the atoms are individually depositing and agglomerating into small islands, then growing together into a complete coating. Deposition is started at a low rate, in a dilute mixture of metal carbonyl in a carrier gas, to permit this phenomenon of the escaping of the carbon monoxide to occur. When the gas has escaped, there is no gap between substrate and coating and a metallurgical bond is achieved.
U.S. Pat. No. 3,086,881 to Jenkin (the inventor herein) describes one method for obtaining an adherent metal coating on a carbon steel or alloy steel substrate. According to the method disclosed in this reference, the iron or steel substrate surface is first deoxidized with hydrogen at a temperature of at least about 700.degree. F. (about 370.degree. C.). An ammonia-carbon monoxide mixture is then flowed into the reactor to displace the hydrogen. Then nickel carbonyl is introduced into the reactor while flow of the ammonia-carbon monoxide mixture continues, resulting in adherent deposition of nickel on the ferrous metal substrate. A disadvantage of this process is that a minimum temperature of about 700.degree. F. is required. Higher minimum temperatures are required when the steel substrate is alloyed with chromium. For example, a minimum temperature of about 900.degree. F. is required for a 1% chrome steel, and a minimum of about 1800.degree. F. is required for a high chrome steel. A further disadvantage is that very high purity gases (H.sub.2, CO,NH.sub.3) are required.
U.S. Pat. No. 3,537,881 to Corwin describes a method for gas plating a substrate which cannot be adherently plated directly. Specifically, a copper surface, which according to patentee cannot be adherently plated with nickel deposited from nickel carbonyl, is first treated with a wetting agent such as mercury (other examples include tin and gold), then the treated substrate surface is vapor coated at 150.degree. C. with nickel deposited from nickel carbonyl vapor. An adherent coating is obtained. Other systems disclosed include zinc substrates wet with copper and then vapor coated with chromium from chromium carbonyl vapor.
Other references disclosing processes in which an intermediate metal layer (or undercoat layer) is applied to a substrate before a metal outer (or outercoat) layer is applied by CVD, include the following: U.S. Pat. No. 2,956,909 to Robinson (plastic or paper substrate; silver intermediate layer and nickel outer layer, i.e., plastic or paper/silver/nickel); U.S. Pat. No. 3,175,924 to Norman et. al. (Vanadium coated on various substrates including nickel plated steel); and U.S. Pat. No. 3,253,946 (Mangenese, chromium, molybdenum or tungsten deposited on various substrates including nickel-plated steel).
U.S. Pat. No. 3,294,654 to Norman et. al. discloses a process which is basically the reverse of the present process; i.e., a substrate which cannot be electroplated is first coated with an intermediate layer by chemical vapor deposition, then a final metal coating is electrodeposited. The non-electroplatable substrate may be highly pure molybdenum or tungsten, or a non-metalic material; the intermediate layer may be a transition metal (Group IVB, VB, or VIB) or aluminum; and the electroplated layer may be copper.
Of various methods of obtaining adherent metal coatings by CVD when the coating metal cannot be adherently deposited directly onto the desired substrate metal in this manner, all presently known processes have some disadvantage. In particular, adherent deposition by CVD from a metal carbonyl onto a ferrous metal substrate has not been previously achieved except by working at high temperatures as for example, in U.S. Pat. No. 3,086,881. High operating temperatures lead to very difficult problems of gas seals and add to the expense of the process; furthermore, it is more difficult to maintain uniform temperature at high operating temperatures than at lower operating temperatures.